/Engine/Source/Runtime/CoreUObject/Public/UObject/PackageTrailer.h
Serializes the trailer from the given archive BUT assumes that the seek position of the archive is at the end of the trailer and so will attempt to read the footer first and use that to find the start of the trailer in order to read the header.
True if a valid trailer was found and was able to be loaded, otherwise false. If the trailer was found but failed to load then the archive will be set to the error state.
bool TryLoadBackwards ( [FArchive](API\Runtime\Core\Serialization\FArchive) & Ar )